Revision as of 19:07, 31 March 2013

Contents

1 Bogre-Banana

1.1 What is Bogre-Banana

Bogre-Banana is a 3D game engine for Haskell. It uses Haskell bindings to the OGRE 3D engine and OIS input system and a library called Reactive-Banana, to create a "Functional Reactive Programming" game engine. Bogre-Banana is designed to be concise and easy to use.

1.2 FRP Crash Course

Programming in FRP consists of creating a network of "Behavior"s and "Event"s (although Events are more like event streams). A Behavior represents something that changes through time. An Event represents discrete time specific events. The key difference between a Behavior and an Event is that a Behavior has a value at all times, while an Event only occurs at specific instances of time.

In the context of a game engine, one might have a stream of events
for keyboard input. When the user presses a key, a corresponding event
is created. The number of times a key has been pressed could be expressed as a
Behavior based off of the keyboard event stream. This Behavior could
be mapped to an output, e.g. displayed on the screen to the user. In a
similar way, time, input, and world state can be expressed as Events and
Behaviors then combined in various ways to create complex interactions
that govern all aspects of the game.

Reactive-Banana is the FRP library used in Bogre-Banana. You can find more information about it at [[1]]. It is highly recommended that you read the Reactive-Banana tutorial [[2]]

1.3 Installation

Bogre-Banana can be installed through the cabal-install tool. There are however some required libraries needed before cabal-install can correctly build Bogre-Banana. For in Ubuntu 12.10 simply run this to install the required packages:

1.5 Tutorial 1: Hello 3D World

function that describes the game. The HookedBogreSystem is needed by many functions in the framework. The SceneManager is mainly used to initialize the world. For now we will just create an empty game:

, it is used by the Reactive-Banana library, and will be in the type of each Behavior and Event. For the most part it can be ignored.

Running this code for the first time will ask you for some graphics settings. Select what you prefer and continue to run the game. You should see a blank window displaying your empty world (you will need to Alt+Tab out of the window to exit).

You may have noticed that the

myGame

function is a

Moment t

monad. In this monad we setup the Behaviors and Events we need for the game. We can also do some IO in this monad. For that we simply use the

liftIO

function. So if we wanted to print out "Hello World!" at the start of the game, we could just make myGame work as follows:

More usefully, one could create an initWorld IO function that will setup the world as needed. Say we want to create a light source and put an ogre head in the world. Make sure to have the mesh files in in the "./Media" directory. The code would now look like this:

Note that the initWorld function makes use of the Graphics.Ogre.HOgre module (the bindings to the Ogre 3D engine). The SceneNode for the ogre head is returned, but at the moment is not used.

So far the code hasn't taken advantage of FRP. Now lets see how we could use Events to print out the current time at each frame. To get the frame Event, we simply use

frameE :: HookedBogreSystem -> Event t BogreFrame

. This returns an Event of BogreFrames. We then have to transform the Event to an Event of the time (of type Float), and then to an an Event of IO actions. We use fmap (actually its infix, "<$>") to transform the events. We intend that whenever the IO Event occurs, it is executed. In Reactive-Banana, this is accomplished by passing the IO Event to the

function to set the position of the ogre head according to a Behavior. We fist convert our frameTimeE from an Event to a Behavior, then transform it into a Behavior of positions. Converting an event to a position can be done with the stepper function which creates a Behavior with an initial value, then updates its value whenever the event occurs. Here is our code now:

Now we are getting the hang of things, why not try and get some input working. Let's try and move the ogre head according to the mouse. here we can simply get the mouse position as a Behavior using the

function to get an Event that occurs whenever the given key is pressed (make sure to import OIS.Types to get the key codes). Let's try and close the window using the escape key (we use the stopBogre function to stop the game):

Note the use of the <$ operator which is like <$> (infix fmap), but it doesn't transform with a function, it simply replaces each event with the left argument.

1.6 Exercises

Create a second ogre head that respond to the mouse in a different way (e.g make the second head mirror the movements of the first) HINT: You can reuse the mousePosB Behavior.

Have the fist ogre head move with the mouse, and the second continuously circle around it. HINT: we already made an ogre head move in a circle around the origin, now make it relative to the first ogre head.

Have the fist ogre head move with the mouse, and the second move directly toward the first with a speed proportional to their distance. HINT: If it is easier to think in terms of velocity, then you may make use of the